The present application relates to an optical element and a method for producing the same. More specifically, the present application relates to an optical element in which a large number of structures are arranged at a fine pitch equal to or less than the wavelength of light for which the amount of reflection is to be reduced.
As shown in FIG. 1, in a lens barrel 101 of an optical instrument such as a camera or a telescope, external light L1 incident from a lens 102 is diffusely reflected at end faces of the lens 102 and a lens 103. Since diffusely reflected light components L2 generated at the end faces travel to behind the lenses 102 and 103, ghosts and flare are caused in an image, resulting in a decrease in the contrast, for example. Consequently, techniques have been used practically, in which a coating is applied onto a surface of a component provided in a lens barrel of an optical instrument such as a camera or a telescope so that the amount of light reflected at the surface of the component provided in the lens barrel is decreased to increase the contrast.
A coating material (produced by LOSIMOL GmbH, Germany, trade name: GT series) that employs ultrafine particles composed of a highly refractive material is widely known as a lens-inner-surface anti-reflection coating used for the inside of a lens barrel (refer to LOSIMOL GmbH, Germany, sole import agency Prince-boueki KK, “Lens-inner-surface anti-reflection coating, GT series”. In order to realize a high optical performance, not only a coating treatment performed on a lens but also a process of light reflected inside a lens barrel in a complex manner is an important point. To suppress the generation of ghosts and flare, it is important to reduce the amount of diffusely reflected light, as described above.
By applying the above lens-inner-surface anti-reflection coating onto a fitting surface or an inclined surface that holds a lens, the amount of reflected light traveling from a lens barrel to the outside or the amount of reflected light traveling into the lens barrel is decreased. The lens-inner-surface anti-reflection coating contains ultrafine particles composed of a highly refractive material. The refractive index of the ultrafine particles is higher than that of glass, and a significant effect of suppressing reflection can be achieved. An excellent anti-reflection effect can be obtained by appropriately selecting the type of lens-inner-surface anti-reflection coating (GT series) used in accordance with the position and the purpose of the application.
According to the above lens-inner-surface anti-reflection coating, a uniform light absorber can be formed because a dye, the particles of which are dispersed on the molecular level, is used. A sufficient effect can be also achieved in preventing inner-surface reflection in highly-refractive glass, which is difficult using a coating that employs a component composed of large particles. FIG. 2 shows measurement results of the above lens-inner-surface anti-reflection coating. Referring to FIG. 2, the inner surface reflectivity is decreased to several percent or less by applying the lens-inner-surface anti-reflection coating.
Furthermore, as a coating material for the purpose of preventing reflection, a graphite coating material (produced by Sanesu Junkatsu Inc., trade name: San coat GR) is known (refer to Sanesu Junkatsu Inc. “Features of San coat GR # series (graphite coating material)”. In this coating material, adjustment ranging from glossy black to completely matt black can be realized.
Furthermore, in displays used in personal computers, car navigation systems, touch panels, and the like, a film on which an anti-reflection or anti-glare treatment has been performed is used in order to prevent reflection of a fluorescent lamp, illumination light, and the like (refer to, for example, Tokushiki Co., Ltd. “Hybrid technology, coating agent, anti-glare coating agent”. The anti-reflection treatment is a technique in which a path of light is adjusted and reflection is suppressed by utilizing a difference in refractive index. The anti-glare treatment is a technique in which light is diffusely reflected by forming projections and recesses on a surface to eliminate glare. For example, the above-mentioned “Hybrid technology, coating agent, anti-glare coating agent” (Tokushiki Co., Ltd. discloses a coating agent for anti-glare films using a dispersion technique of fillers.
As a technique other than the coating techniques described above, a technique in which an anti-reflection structure is formed in a lens barrel of an optical instrument such as a camera or a telescope is known (refer to, for example, Japanese Unexamined Patent Application Publication No. 2006-293093).
FIG. 3A is a schematic view showing an arrangement of a zoom lens system included in a lens barrel. This zoom lens system is a lens system in which zooming is performed by moving a plurality of lens groups in an optical axis direction. This zoom lens system includes a first lens group G1 located at a position closest to an object, a second lens group G2 that is located at a position second closest to the object and that has a power different from that of the first lens group G1, and a third lens group G3 located at a position third closest to the object. A plurality of anti-reflection structures 111 are provided on a portion of the inner surface of a lens barrel, the portion corresponding to a range from the first lens group G1 to a position on the imaging surface S side. The anti-reflection structures 111 are periodically arranged in the form of an array at a pitch smaller than the shortest wavelength of incident light. Since the anti-reflection structures 111 are provided on the inner surface of the lens barrel in this manner, reflection of off-axis light in the lens barrel, the off-axis light being incident on the lens groups, can be suppressed. Thus, a lens barrel in which degradation of image quality due to unnecessary light is suppressed can be provided.
FIG. 3B is a schematic enlarged view of the second lens group G2 of the lens barrel. As described above, in the second lens group G2, a positive lens 112 and a positive lens 113 are disposed with a certain distance therebetween. Accordingly, off-axis light that does not contribute to the formation of an object passes through the first lens group G1, is incident on the second lens group G2, and then reaches a portion of the inner circumferential surface of the lens barrel, the portion corresponding to the above certain distance. When the off-axis light reaches and is reflected from the inner circumferential surface of the lens barrel, the reflected light becomes stray light and reaches the imaging surface S, thus causing flare and ghosts. To prevent this problem, as shown in FIG. 3B, in the second lens group G2 of the lens barrel, the anti-reflection structures 111 are provided on the portion of the inner circumferential surface of the lens barrel, the portion corresponding to the above certain distance.
Here, the anti-reflection structures 111 are composed of structural units each having a certain shape and periodically arranged in the form of an array at a pitch smaller than the lower limit of the wavelength of incident light, that is, at a pitch smaller than the shortest wavelength of the incident light. By periodically arranging the structural units each having a certain shape in the form of an array in this manner, the refractive index to incident light is apparently continuously changed to form an anti-reflection functional surface in which the incident angle dependence and the wavelength dependence of transmission/reflection characteristics at the interface with an air layer are small. As for an example of the anti-reflection structures 111, as shown in FIG. 3C, circular-cone-shaped projections each having a height H are used as the structural units, and these circular-cone-shaped projections are periodically arranged in the form of an array at a pitch P.